Superior mitochondrial adaptations in human skeletal muscle after interval compared to continuous single-leg cycling matched for total work.
Level 2 - randomized trial
Randomized within-subject bilateral comparative trial
PubMed 27396440 · doi:10.1113/JP272570
What was done
Ten young active men completed a 2-week intervention comprising six training sessions using a counterweighted single-leg cycling model. Within each participant, one leg was randomly assigned to high-intensity interval training (HIIT: 4 × [5 min at 65% peak power and 2.5 min at 20% peak power]) and the contralateral leg to moderate-intensity continuous training (MICT: 30 min at 50% peak power), performed 10 minutes apart in alternating order. Total work per session was matched between protocols (MICT: 143 ± 8.4 kJ vs. HIIT: 144 ± 8.5 kJ, P > 0.05). Unilateral graded-exercise tests and muscle biopsies were performed to measure single-leg peak oxygen uptake (V̇O2, peak), citrate synthase (CS) maximal activity, mitochondrial respiration across respiratory complexes, and whole-muscle and fibre-specific mitochondrial protein abundance (COXIV, NDUFA9, MFN2).
What was found
Post-training CS maximal activity was higher following HIIT compared to MICT (10.2 ± 0.8 vs. 8.4 ± 0.9 mmol kg protein⁻¹ min⁻¹, interaction P < 0.05). Mass-specific oxidative phosphorylation capacities were also significantly greater after HIIT than MICT for complex I (23.4 ± 3.2 vs. 17.1 ± 2.8 pmol O2·s⁻¹·mg wet weight⁻¹) and complexes I and II combined (58.2 ± 7.5 vs. 42.2 ± 5.3 pmol O2·s⁻¹·mg wet weight⁻¹; interaction P < 0.05). Intrinsic mitochondrial function normalized to CS maximal activity did not change (P > 0.05). Whole-muscle protein content of COXIV (+24%), NDUFA9 (+11%), and MFN2 (+16%) increased across training with no difference between legs (P < 0.05). Training did not alter single-leg V̇O2, peak or fibre-type-specific protein content (P > 0.05).
Why it matters
Using a within-subject model that eliminates systemic confounding and isolates muscular responses, this study demonstrates that exercise intensity and interval contraction patterns drive superior skeletal muscle mitochondrial adaptations independently of total work volume.
Limits
The study is limited by a very small sample size (n = 10) restricted entirely to young, active males. The intervention lasted only 2 weeks, and the single-leg cycling paradigm does not reflect systemic whole-body cardiovascular responses or long-term functional performance outcomes.
Cited by
- supports For volume-matched exercise protocols, high-intensity interval training induces greater mitochondrial biogenesis than moderate-intensity continuous exercise.